FIG. 1 is a block diagram for illustrating entities for segmenting and reassembling source data unit (SDU) packets in a medium access control (MAC) layer in accordance with the related art.
Segmentation is performed at a transmitting side. That is, a segment entity segments a source data unit (SDU) into packets each having a transmittable size based on a corresponding option of an adaptive modulation and coding (AMC).
Herein, the SDU is a packet received from a higher layer, i.e., an Internet protocol layer and a processing data unit (PDU) denotes each piece of segmented SDUs having a transmittable size by the AMC.
FIG. 2 is a graph showing a signal-to-noise ratio (SNR) and a bit error rate (BER) corresponding to options of the AMC.
Referring to FIG. 2, in case of a service requiring 10−6 BER, the option 1 of the AMC is selected for a 5 dB SNR and the option 2 of the AMC is selected for 7 dB of SNR. If a quadrature phase shift keying (QPSK) is used as a modulation scheme with a channel coding rate of ⅙ in the AMC option 1, a single carrier wave can transmit ⅓ bit. If the QPSK is used as the modulation scheme with a channel coding rate of ⅓ in the AMC option 2, a single carrier wave can transmit ⅔ bit. Since a wireless channel condition varies according to a speed and a direction of a channel, the AMC option must be properly selected according to the channel conditions in order to transmit data with satisfying the required BER.
FIG. 3 is diagram showing transmission and retransmission of a segmented SDU in a channel having constant environment according to the time.
Since a wired channel has a constant channel environment, a data transfer rate of a lower layer is fixed when a service is started. For example, if 100 byte of data can be transmitted in a single transmission period, the segment entity in the transmitting size divides a SDU inputted from the higher layer by a unit of 100 byte. That is, the SUD is divided into a plurality of PDUs each having 100 bytes. Then, a sequence number (SN) is assigned to each of the PDUs in order. If an error is generated while transmitting a particular PDU, a corresponding PDU is retransmitted using the assigned sequence number.
The PDUs are transferred to a lower layer, i.e., a physical layer, and then transmitted to a receiving side through a wireless channel. As shown in FIG. 1, the receiving side includes a SDU buffer, a reassembling entity, a PDU inserting entity, and a PDU buffer. The PDU inserting entity orderly inserts the received PDUs to the PDU buffer according to the SN of the PDU. The reassembling unit reassembles the SDU through rearranging the received PDUs based on the SN and segmentation control (SC) identification included in a header of the PDU. The reassembled SDU is transferred to a higher layer. The SC identification has options such as No, First, Continue and Last. The No option denotes un-segmented SDU, the option of First denotes a first PDU, the Continue option denotes intermediate PDUs and the Last option denotes the last PDU. Such options are correspondently set in the PDU header, and the SC identification is not changed when a PDU is retransmitted.
As described above, a SDU is configured of PDUs having sequence numbers (SN) with corresponding SC identifications, such as the First option for the first PDU, the Continue option for intermediate PDUs and the Last option for the last PDU. The receiving side transmits an acknowledge (ACK) signal to the transmitting side in order to notice normal receipt of the transmitted PDUs when the receiving side normally receives PDUs.
If a channel environment varies according to the time, for example, a wireless channel environment, the number of data to be transmitted varies too.
FIG. 4 is a view showing why it is impossible to retransmit segmented SDUs in a channel having variable environment according to the time.
The size of the PDU is not changed when the PDUs are transmitted through the wired channel that has the constant condition as shown in FIG. 3. However, the size of the PDU is changed from time to time in a wireless channel that has variable conditions. That is, the AMC options vary according to the conditions of the wireless channel which is changed from time to time. Consequently, a data transfer rate is changed according to the variation of the AMC options.
As shown FIG. 4, if transmitting 200 byte of data fails at a first transmission period T1, retransmission is tried at a fourth transmission period T4. If the wireless channel environment at the T4 is worse than that at the T1, that is, the wireless channel environment at the T4 allows only 100 byte data to be transmitted, a lower AMC option is allocated. As a result, it is impossible to retransmit the 200 byte data at the T4.
Therefore, the PDUs must be segmented again into PDUs of 10 byte each and corresponding new SNs must be assigned to new PDUs. However, if the PDUs are successfully transmitted in the transmission periods T2 and T3, it is impossible to reassign the SNs. That is, it must wait for another transmission period having a channel environment that allows 200 byte of data to be transmitted for retransmitting the 200 byte of PDU without and reassigning SN. Therefore, transmission of the SDU is delayed.
In order to improve such a drawback, a unit size of PDU is determined corresponding to a minimum AMC option and the SDU is segmented and the SNs are assigned to each of the PDUs based on the determined unit size, as shown in FIG. 5. The SDU is segmented into more numbers of PDUs of smaller size. However, when the higher AMC option is allowed in a particular transmission period because of good channel environment, many PDUs are transmitted in the single transmission period. In this case, many PDU headers are also transmitted with the PDUs in the single transmission period. Therefore, it may degrade transmission efficiency. Furthermore, if the SDU is segmented into PDUs with the minimum AMC option, a length of an identification field for a sequence number becomes longer. As a result, the required length of the header also becomes longer. It also degrades the transmission efficiency.